[SCG56-P14] Silicate inclusions in dispersed chromites at the mantle-crust transition zone: the link between dunite and chromitite formations
キーワード：Oman ophiolite, dunitic mantle-crust transition zone, chromite, silicate inclusions, fluid-melt-rock reactions
Melt-rock reactions have a significant impact on the mineralogical and chemical composition of mantle peridotite. Observations in ophiolites and more patchy data from oceanic drilling show that below oceanic spreading centres, MORB-peridotite interaction at the transition between the mantle and the oceanic crust leads to the formation of a thick dunitic layer hosting chromitites ore bodies. Dunite and chromitite appear then to be two companion products of a common melt-peridotite reaction.
The petrological processes that originate the dunitic mantle-crust transition zone (DTZ) can be studied in the Maqsad area (Oman ophiolite) where is exposed a paleo-mantle diapir that fed the former spreading centre with MOR-like melts. The DTZ reaches more than 300 m in thickness atop the central part of the paleo-diapir. Interstitial melt suffered crystallization during its migration, comprising mainly plagioclase, clinopyroxene, but also orthopyroxene and pargasitic amphibole. The presence of such minerals witness migration of a hydrous melt that is restricted to this level and does not reach deeper levels in the mantle section. It was tentatively attributed to a parent melt hybrid between the MORB generated by the deep partial melting of the mantle and a hydrated fluid fraction coming from above, likely seawater in origin (Rospabé et al., 2017, Geology).
The presence of orthopyroxene and amphibole, together with mica, enclosed in chromite grains (scattered in dunites and in chromitites) strongly suggests that this hybrid melt was involved early in both the formation of dunite and the re-mobilization of chromium at the Maqsad mantle-crust transition zone (Borisova et al., 2012, Journal of Petrology; Rospabé et al., 2017, Geology). By the detailed and systematic study of the silicate inclusions enclosed in the chromite grains dispersed in the DTZ, we show they are an excellent target to make the link between the dunite and the chromitite ore bodies of higher grade. Our results suggest that the ore genesis processes involved in the formation of chromitites are not restricted to the ore bodies themselves but operated on the much larger scale of the whole DTZ. It leads to consider the entire DTZ as an ore deposit. In this spirit, the DTZ dunites may be regarded as low grade dispersed ore.
The petrological processes that originate the dunitic mantle-crust transition zone (DTZ) can be studied in the Maqsad area (Oman ophiolite) where is exposed a paleo-mantle diapir that fed the former spreading centre with MOR-like melts. The DTZ reaches more than 300 m in thickness atop the central part of the paleo-diapir. Interstitial melt suffered crystallization during its migration, comprising mainly plagioclase, clinopyroxene, but also orthopyroxene and pargasitic amphibole. The presence of such minerals witness migration of a hydrous melt that is restricted to this level and does not reach deeper levels in the mantle section. It was tentatively attributed to a parent melt hybrid between the MORB generated by the deep partial melting of the mantle and a hydrated fluid fraction coming from above, likely seawater in origin (Rospabé et al., 2017, Geology).
The presence of orthopyroxene and amphibole, together with mica, enclosed in chromite grains (scattered in dunites and in chromitites) strongly suggests that this hybrid melt was involved early in both the formation of dunite and the re-mobilization of chromium at the Maqsad mantle-crust transition zone (Borisova et al., 2012, Journal of Petrology; Rospabé et al., 2017, Geology). By the detailed and systematic study of the silicate inclusions enclosed in the chromite grains dispersed in the DTZ, we show they are an excellent target to make the link between the dunite and the chromitite ore bodies of higher grade. Our results suggest that the ore genesis processes involved in the formation of chromitites are not restricted to the ore bodies themselves but operated on the much larger scale of the whole DTZ. It leads to consider the entire DTZ as an ore deposit. In this spirit, the DTZ dunites may be regarded as low grade dispersed ore.